1. Field of the Invention
This invention relates to a lens evaluation device and, more particularly, to a device for measuring the optical transfer function (OTF) of the lens for evaluating lens characteristics.
2. Description of the Related Art
A semiconductor exposure device has hitherto been employed for formation of a circuit pattern of a semiconductor device. With such semiconductor exposure device, a reticle on which a circuit pattern for an original picture corresponding to the circuit pattern to be formed on the semiconductor device is formed is illuminated by a light beam radiated from an exposure light source. An image of the circuit pattern of the original picture obtained by illuminating the reticle with the light beam is formed via a projecting lens on a semiconductor wafer (semiconductor device) for exposing the circuit pattern corresponding to the image of the circuit pattern of the original picture on a resist layer formed on the semiconductor wafer.
FIG. 1 shows an arrangement of this type of a conventional semiconductor exposure device.
Referring to FIG. 1, the semiconductor exposure device includes an exposure light source 131 for radiating a light beam for illuminating a reticle 133 on which is formed a circuit pattern for an original picture corresponding to a circuit pattern to be formed on the semiconductor device. As the exposure light source 131, an excimer laser or an ultra-high pressure mercury arc lamp radiating the light of a shorter wavelength is employed. The light beam emanated from the exposure light source 131 is incident on an illuminating optical system 132 by means of which it is radiated to the reticle 133 on which there is formed the circuit pattern of the original picture corresponding to the circuit pattern formed on the semiconductor device. The image of the circuit pattern of the original picture is contracted in size to, for example, a one-fifth size, by a contracting projecting lens 113, so as to be projected on a resist layer formed on a semiconductor wafer 114, so that light exposure is made on the resist layer to a shape corresponding to the image of the circuit pattern of the original picture.
The semiconductor wafer 114 is set on an XY stage 115. The XY stage 115 is moved along two mutually perpendicular axes for variably adjusting the position of the semiconductor wafer 114 set thereon for achieving relative position setting (registration) between the reticle 133 and the semiconductor wafer 114 in order to get the image of the circuit pattern of the original picture on the reticle 133 correctly formed at a pre-set position on the semiconductor wafer 114. Such relative position setting between the reticle 133 and the semiconductor wafer 114 is achieved by detecting position setting marks formed on the semiconductor wafer 114 and moving the XY table 115 in a controlled manner responsive to the detection output.
For producing a fine circuit pattern of the semiconductor device to high accuracy, it is necessary for the image of the circuit pattern of the original picture formed on the reticle to be formed with high resolution on the semiconductor wafer. For forming the image of the circuit pattern of the original picture with high resolution and exposing the resist layer on the semiconductor wafer to light, it is necessary to employ not only a light source capable of radiating a light beam of an extremely short wavelength as a light source for exposure, but also a lens having extremely high resolution characteristics as a lens forming the image of the circuit pattern of the original picture on the semiconductor wafer.
Thus the image-forming lens employed in the semiconductor exposure device has to be evaluated as to whether or not it is capable of forming an image of an extremely fine circuit pattern having a linewidth on the order of 0.25 .mu.m with high accuracy on the resist layer of the semiconductor wafer. If the image-forming lens employed in the semiconductor exposure device is not capable of forming the image of the fine circuit pattern on the semiconductor wafer with high accuracy, or if lens characteristics are deteriorated through the use of the semiconductor exposure device, it becomes necessary to exchange the lens with a lens having satisfactory characteristics.
The method for evaluating the characteristics, such as resolution, of a lens employed for forming the image of the circuit pattern of the original picture on the semiconductor wafer, employs a semiconductor exposure device constructed as shown in FIG. 1, in which a reticle having lines and spaces formed thereon as the reference of resolution is illuminated by a light beam outgoing from an exposure light source. An illuminated image of the line and spaces, thus formed on the reticle, is formed on the resist layer on the semiconductor wafer set on the XY stage via an image-forming lens which is to be evaluated. This resist layer is exposed and the exposed portion is developed. The cross-sectional shape of the pattern on the semiconductor wafer thus developed is measured using a scanning electron microscope. The exposure light volume distribution is calculated from the measured values of the cross-sectional shape and sensitivity characteristics of the resist layer on the semiconductor wafer. The optical transfer function of the image-forming lens is calculated form the calculated values of the exposure light volume distribution.
The above-described method is described in "Technology of Semiconductor Lithography" by Koichiro Otori, published by SANGYO TOSHO Company, page 93.
The reason the above-described method is employed in evaluating optical characteristics of the image-forming lens employed in the semiconductor exposure device is that the light beam radiated from the exposure,light source employed in the semiconductor exposure device is low in coherence and it is extremely difficult for such light beam to be converged to form a point light source for evaluating lens characteristics.
With the above-described method, consisting in developing an image of line and spaces formed on the reticle as the resolution reference on the resist layer on the semiconductor wafer, and evaluating lens characteristics from the developed picture, a series of measurement processes including a series of light exposure processes employing the semiconductor exposure device become complex. On the other hand, the operation of calculating the characteristics of the lens under measurement is a time-consuming operation, such that the lens characteristics cannot be known promptly.
In addition, the process for producing the light exposure volume distribution using sensitivity characteristics of the photoresist formed on the semiconductor wafer is susceptible to errors.
Furthermore, with the conventional semiconductor exposure device, since it is difficult to achieve highly accurate relative position setting between the reticle having lines and spaces as resolution reference formed thereon and the semiconductor wafer on which the image of these lens and spaces is to be formed, the image of the lines and spaces cannot be formed with high accuracy on the resist layer on the semiconductor wafer, as a result of which the characteristics of the lens under measurement cannot be obtained correctly.